Many coal liquefaction processes which are at an advanced stage of development operate in the liquid phase and are designed to produce both heavy liquids and solids at room temperature. In the case of liquefaction by extraction, the fluid phase is viscous, liquefied coal with some solid particles suspended in it.
Extraction of the organic matter in coal with supercritical solvents allows the upgrading of a fraction of the original coal into an organic having only a slight fraction of inorganic material.
The current state of the art is exemplified in the publication of R. R. Maddocks et al. in Chemical Engineering Progress (CEP), June, 1979 in pages 49-55. Maddocks et al. reported that the extraction of coal with supercritical toluene at 319.degree. C. (606.degree. F.) provide a 21-weight-percent yield of extract having a low ash content. It is further suggested that this process would be commercially attractive if yields of extract could be increased. Other studies have shown that the economics of coal extraction rapidly rise as the extraction and efficiency increases from 20 to 40%.
It would be expected that oral extraction efficiency under supercritical conditions increases with temperature. However, attempts to extract coal with efficient aromatic solvents such as toluene at temperatures above 400.degree. C. actually result in lower yield of liquid extract due to gasification coking with perhaps some polymerization reactions to yield an increasing percentage of insoluble residue. Furthermore, more expensive materials of construction and an energy penalty are required for a high-temperature reactor and process. Other examples of low yield, supercritical solvent extraction processes are disclosed in U.S. Pat. Nos. 3,558,468 and 3,607,717 which illustrate the use of an aromatic solvent such as benzene at supercritical conditions. U.S. Pat. No. 3,607,716 utilizes phenanthrene to process liquid coal extracts under supercritical conditions, and U.S. Pat. No. 4,036,731 discloses a mixture of an aromatic solvent and a hydrogen donor solvent such as tetralin, preferably in the presence of hydrogen gas to extract coal under supercritical conditions.